1. Field of the Invention
The present invention relates to an LED drive circuit that directly drives an LED (light-emitting diode) by use of a voltage obtained by rectifying alternating current power, and to an LED illumination component, an LED illumination device, and an LED illumination system that use the LED drive circuit.
2. Description of the Prior Art
An LED is characterized by its low current consumption, long life, and so on, and its range of applications has been expanding not only to displays but also to illumination apparatuses and the like. An LED illumination apparatus often uses a plurality of LEDs in order to attain desired illuminance.
A general-use illumination apparatus often uses a commercial AC 100 V power source, and considering, for example, a case where an LED illumination component is used in place of a general-use illumination component such as an incandescent lamp, it is desirable that, similarly to a general-use illumination component, an LED illumination component also be configured to use a commercial AC 100 V power source.
Furthermore, in seeking to perform light control of an incandescent lamp, a phase-control light controller (referred to generally as an incandescent light controller) is used in which a switching element (generally, a thyristor element or a triac element) is switched on at a certain phase angle of an alternating current power source voltage and that thus allows light control through control of power supply to the incandescent lamp to be performed easily with a simple operation of a volume element (see, for example, JP-A-2005-26142).
It is desirable that in seeking to perform light control of an LED illumination component that uses an AC power source, the LED illumination component be connectable as it is to an existing phase-control light controller for an incandescent lamp. By changing only an illumination component from an incandescent lamp to an LED illumination component while using existing light control equipment therewith, compared with a case of using the incandescent lamp, power consumption can be reduced considerably (see, for example, JP-A-2006-319172). Furthermore, this can also secure compatibility without requiring the light control equipment to be changed to a type used exclusively for an LED illumination component and thus reduces equipment cost. Furthermore, an LED illumination apparatus takes any of many various forms such as a lamp for main illumination, an electric bulb, a downlight, an under-shelf light, and a lamp for indirect illumination and uses a power source technique suitable for the form it takes.
Examples of such a power source technique include an AC/DC method in which an LED is driven by use of a DC voltage obtained by smoothing AC power and an AC direct drive method in which an LED is driven directly by use of a voltage obtained by rectifying AC power. The methods as the power source techniques have their respective characteristics, and there are two types of the AC/DC method: a voltage step-up type and a voltage step-down type. Either of these types, while allowing high-efficiency driving of an LED, involves driving an LED by use of a DC voltage obtained by smoothing an alternating current voltage with a voltage smoother, which leads to the complication of a circuit and requires that a transformer, a coil, and a capacitor having large time constants be used selectively and thus that components having relatively large volumes be used.
On the other hand, in the AC direct drive method, while this method is somewhat less efficient compared with the AC/DC method, an LED is turned off if a rectified input voltage is smaller than a forward voltage obtained when the LED starts to glow. The LED is turned off in repeated cycles of 100 Hz to 120 Hz obtained by rectifying a frequency of 50 Hz to 60 Hz of a general-purpose power source. In a case of a camera or the like, if this timing synchronizes with imaging timing of the camera, a large variation in brightness is perceived, which, however, is hardly perceivable to the human eye due to an extremely short blinking cycle. Also, this method involves directly driving the LED by use of a rectified voltage, thus providing a relatively simple configuration including a reduced number of components and requiring no high-profile components such as a coil and a capacitor, and is therefore used favorably for a thin power module. For example, in a case of an illumination apparatus such as an under-shelf light, a power module that takes up only a limited space is required, and thus it is best to use the AC direct drive method.
Now, FIG. 14 shows a configuration of a conventional incandescent lamp illumination system. The incandescent lamp illumination system shown in FIG. 14 includes a phase-control light controller 2, a diode bridge DB1, and an incandescent lamp 41. FIG. 20 shows a configuration example of the phase-control light controller 2, in which a resistance value of a variable resistor Rvar1 is made to vary, and a triac Tri1 is thus switched on at a power source phase angle depending on the resistance value. Typically, the variable resistor Rvar1 is built in the form of a rotary knob or a slider and so configured that changing an angle of rotation of the knob or the position of the slider allows light control of an illumination component. Moreover, in the phase-control light controller 2, a capacitor C1 and an inductor L1 constitute a noise suppression circuit that reduces noise fed back into an alternating current power source line from the phase-control light controller 2.
FIG. 16 shows as one example voltage and current waveforms at various parts of the system in a case where the incandescent lamp 41 is driven while being light-controlled by the phase-control light controller 2. In FIG. 16, there are shown a waveform of an output voltage V1 of the phase-control light controller 2, a waveform of a voltage V41 across the incandescent lamp 41, and a waveform of a current I41 flowing through the incandescent lamp 41. When the triac Tri1 included in the phase-control light controller 2 is switched from an off-state to an on-state, the voltage V41 across the incandescent lamp 41 increases sharply, and thus the current I41 flowing through the incandescent lamp 41 also increases sharply, so that the incandescent lamp 41 is turned on. After that, during the time the triac Tri1 is on, the current continues to flow through the incandescent lamp 41, and the turned-on state of the incandescent lamp 41 is thus maintained as long as the output voltage V1 of the phase-control light controller 2 has a value higher than around 0 V.
It is known, however, that also in performing light control of the incandescent lamp 41 with the phase-control light controller 2 as shown in FIG. 14, the use of a low-wattage incandescent lamp as the incandescent lamp 41 leads to the occurrence of flickering and blinking, making it impossible to perform the light control properly. The output voltage of the light controller rises at a threshold voltage of the triac Tri1 included in the phase-control light controller 2. This rising timing varies considerably in response to fluctuations of an alternating current power source 1, so that a light control phase angle varies. When the light amount is low, the ratio of the amount of this variation in phase angle increases, which leads to the occurrence of flickering.
It is desired that in seeking to perform light control of an LED illumination component that uses an alternating current power source, a phase-control light controller be used as in a case of performing light control of an incandescent lamp. Now, FIG. 15 shows a conventional example of an LED illumination system capable of performing light control of an LED illumination component that uses an alternating current power source. The LED illumination system shown in FIG. 15 includes a phase-control light controller 2, a diode bridge DB1, an LED module 3, a current limitation circuit 4, and a drive portion 5. FIG. 17A shows waveforms of a voltage V2 generated at a positive side output end of the diode bridge DB1 and a current ILED of the LED module 3 in a case where a light control level is set to a high brightness level, and FIG. 17B shows those in a case where the light control level is set to a low brightness level.
In a case where the light control level is set to a high brightness level, a triac Tri1 included in the phase-control light controller 2 is switched from an off-state to an on-state at a small phase angle (for example, 40°) to cause the voltage V2 generated at the positive side output end of the diode bridge DB1 to rise sharply (see FIG. 17A), upon detection of which the drive portion 5 starts passing a current through the LED module 3, so that the LED module 3 is turned on. After that, the current flowing though the LED module 3 is controlled so as to be constant by the current limitation circuit 4, and the turned-on state of the LED module 3 is thus maintained during the time a voltage across the LED module 3 is higher than a forward voltage obtained when the LED module 3 starts to glow. Furthermore, in a case where the light control level is set to a low brightness level, the triac Tri1 is switched from the off-state to the on-state at a large phase angle (for example, 130°) to cause the voltage V2 generated at the positive side output end of the diode bridge DB1 to rise sharply (see FIG. 17B), so that the LED module 3 is turned on.
FIG. 18 shows a VF-IF curve (relationship between a forward voltage and a forward current) of each of the incandescent lamp 41 and the LED module 3. Each of the incandescent lamp 41 and the LED module 3 is driven by use of a constant current (I4a, Ia), and a comparison between these cases indicates that during a time period in which an applied forward voltage is high (Vf>V4a, Va), a predetermined current (I4a, Ia) flows through each of the incandescent lamp 41 and the LED module 3, whereas during a time period in which the applied forward voltage is low (Vf<V4a, Va), based on the relationships shown in FIG. 18, the constant current (14a, Ia) can no longer be passed, and thus there occurs a decrease in current flowing through each of the incandescent lamp 41 and the LED module 3. For example, at a certain forward voltage (V4b, Vb), a current (I4b, Ib) is obtained. Now, FIG. 19 shows temporal changes in forward voltage applied to the LED module 3 and in current in the LED module 3. In a case where the light control level is set to a low brightness level and the phase angle is large, for example, in FIG. 19, when the forward voltage rises at timing t1, the current in the LED module 3 has a value I1. Then, when, after the occurrence of a variation Δtj in phase angle from the timing t1 to timing t2, the forward voltage rises at the timing t2, the current in the LED module 3 has a value I2. Based on the VF-IF curve of the LED module 3 shown in FIG. 18, with the forward voltage having a value Va or lower, the current in the LED module 3 decreases abruptly, and thus a variation ΔIj in current in the LED module 3 with respect to the variation Δtj in phase angle is large.
With the alternating current power source 1 having a frequency of 50 Hz to 60 Hz, when a light-emitting element is driven directly by use of a voltage rectified by the diode bridge DB1, blinking occurs repeatedly at 100 Hz to 120 Hz, which, however, is too fast for the human eye to follow and thus is perceived as if the light-emitting element is glowing continuously. In order to maintain brightness at a constant level, it is required that the current in the LED module 3 be set to have a constant value in every cycle. Generally speaking, however, various devices are connected to the alternating current power source 1, so that an output voltage of the alternating current power source 1 fluctuates in various cycles. As a result, there occurs a variation in switching timing of the triac Tri1 included in the phase-control light controller 2 to cause a minute variation in phase angle. In a case where the light control level is set to a low brightness level, this results in a large variation in current in the LED module 3, and when alternating current power fluctuates at a low frequency (for example, a little higher than 10 Hz or lower), such a variation can be followed by the human eye and thus is perceived in the form of flickering.
Furthermore, the amount of the above-described variation is relatively small when a light emission duration of the LED module 3 is long and relatively large when the light emission duration of the LED module 3 is short. For example, if the switching timing of the triac Tri1 varies by 40 μs, at a phase angle of 30°, the amount of the variation is substantially 1%, i.e. there occurs no noticeable degree of change in light (luminance), whereas at a phase angle of 130° or larger, there occurs a noticeable degree of change in light (luminance).